1. Field of the Invention
The present invention relates to a novel nickel powder suitable for use in a thick-film conductor paste and a process for preparing the same, and further relates to a conductor paste prepared using this nickel powder and multilayer electronic components having conductor layers formed using this paste.
2. Description of the Prior Art
Thick-film pastes such as a conductor paste and a resistor paste are used in order to produce components such as an electric circuit, a resistor, a capacitor and an IC package in the electronics field. They are pastes prepared by homogeneously mixing and dispersing a conductive powder of a metal, an alloy, a metal oxide or the like, if necessary, together with a vitreous binder and other additive(s) in an organic vehicle, and they are respectively formed into a conductor film and a resistor film by applying the same onto a substrate and then firing at a high temperature.
Ceramic multilayer electronic components, such as a multilayer capacitor and a multilayer inductor, and ceramic multilayer substrates are generally produced by laminating a plurality of unfired ceramic green sheets of a dielectric material, a magnetic material or the like alternately with a plurality of layers of an inner conductor paste, and then simultaneously firing them at a high temperature. As the mainstream inner conductors, noble metals such as palladium, silver-palladium, and platinum have hitherto been used. In aspects of saving resources and satisfying the requirement of improvements in respect of delamination, cracking, etc. attributed to the oxidative expansion of palladium or silver-palladium during firing thereof, however, base metal materials such as nickel have recently attracted attention.
In these multilayer components and multilayer substrates, there is a tendency to increase the number of layers to be laminated. For example, a multilayer capacitor having hundreds of laminated layers has come to be produced. In view of this, ceramic layers and accordingly inner conductor layers are required to be made in the form of a thinner film. For example, when the thickness of a ceramic layer is reduced to about 3 .mu.m, the thickness of an inner conductor layer must be reduced to at most 1 .mu.m, desirably about 0.5 .mu.m, because otherwise the middle portion of the resulting laminate is so thickened as to cause structural defects and lowered reliability.
In the case of a conductor paste prepared using an ordinary nickel powder, however, there is a limit for the formation of thinner films therefrom because it poses not only such a problem that the resulting inner conductor yields a discontinuous film due to oversintering in the course of firing and thus increases the resistance value or undergoes disconnection, but also such a problem that the thickness of the conductor increases as a result of the aggregation of the nickel powder. More specifically, the nickel powder, when fired in an inert atmosphere or in a reducing atmosphere particularly for the purpose of preventing oxidation thereof, is quickly sintered and begins to sinter and shrink at a temperature as low as 400.degree. C. or below even when it is a single crystal powder comparatively low in activity. On the other hand, the sintering-starting temperature of a ceramic layer is generally far higher than that temperature. For example, it is about 1,200.degree. C. for barium titanate, which therefore does not shrink together with nickel films even by co-firing and allows the nickel films to be pulled in the face direction. Thus, it is conceivable that small voids formed in the nickel films by sintering at a comparatively low temperature are expanded in keeping with the progress of sintering in a higher temperature region and tend to become large, while the films are liable to grow in the thickness-wise direction thereof due to the aggregation of the nickel powder.
In order to make nickel inner conductor layers thinner, therefore, it is thought necessary to render a nickel powder so fine and better in dispersibility as to hinder the formation of voids in the course of firing, and to make the sintering shrinkage behavior of these layers agree with that of ceramic layers.
The disagreement in sintering shrinkage behavior between the conductor layers and the ceramic layers is so causative of structural defects such as delamination and cracking as to lower the yield and the reliability particularly when the films are thick.
Various investigations have hitherto been made for the purpose of suppressing the sintering of a conductor till the sintering-starting temperature of ceramic layers. For example, the addition of a large amount of a ceramic powder having the same composition as that used in ceramic layers can retard apparent shrinkage of conductor layers up to around 800.degree. C. Since the sintering of a metal powder itself is not suppressed, however, the continuity and conductivity of conductor layers are consequently spoiled when firing is conducted at a temperature as high as about 1,300.degree. C.